HAV (Hepatitis A Virus) is an RNA virus classified as hepatovirus which can cause Hepatitis, an acute infection of the liver. HAV is usually spread by a fecal route, transmitted person-to-person by ingestion of contaminated food or water or through direct contact with an infectious person.
Hepatitis A has an incubation period of about four weeks. The virus replicates in the liver. Relatively large quantities of virus are shed in the feces during the incubation period before the onset of clinical symptoms, and a brief viremia occurs. The severity of illness ranges from the asymptomatic to anicteric or icteric hepatitis. The virus is non-cytopathic when grown in cell culture. Its pathogenicity in vivo, which involves necrosis of parenchymal cells and histiocytic periportal inflammation, may be mediated by cellular immune responses. By the time of onset of symptoms, excretion of virus in the feces has declined and may have ceased and anti-HAV IgM increases in titer. Anti-HAV IgG may be detected one to two weeks later and persists for years. The HAV genome comprises about 7,500 nucleotides (nt) of positive sense RNA which is polyadenylated at the 3′ end and has a polypeptide (VPg) attached to the 5′ end. A single, large open reading frame (ORF) occupies most of the genome and encodes a polyprotein with a theoretical molecular mass of Mr 252,000. The HAV polyprotein is processed to yield the structural (located at the amino-terminal end) and non-structural viral polypeptides. Many of the features of replication of the picornaviruses have been deduced from studies of prototype enteroviruses and rhinoviruses, in particular poliovirus type 1.
Hepatitis A virus enters the body by ingestion and intestinal infection. The virus then spreads, probably by the bloodstream, to the liver, a target organ. Large numbers of virus particles are detectable in feces during the incubation period, beginning as early as 10-14 days after exposure and continuing, in general, until peak elevation of serum aminotransferases. Virus is also detected in feces early in the acute phase of illness, but relatively infrequently after the onset of clinical jaundice. Interestingly, antibody to hepatitis A virus that persists is also detectable late in the incubation period, coinciding approximately with the onset of biochemical evidence of liver damage. Hepatitis A antigen has been localized by immunofluorescence in the cytoplasm of hepatocytes after experimental transmission to chimpanzees. The antigen has not been found in any tissue other than the liver following intravenous inoculation.
Various serologic tests are available for hepatitis A, including immune electron microscopy, complement-fixation, immune adherence hemagglutination, radioimmunoassay, and enzyme immunoassay. Immune adherence hemagglutination, which had been widely used, is moderately specific and sensitive. Several methods of radioimmunoassay have been described; of these, a solid-phase type of assay is particularly convenient, very sensitive, and specific. Very sensitive enzyme immunoassay techniques are used widely. Only one serotype of hepatitis A virus has been identified in volunteers infected experimentally with the MS-1 strain of hepatitis A, in patients from different outbreaks of hepatitis in different geographic regions, and in random cases of hepatitis A. Isolation of virus in tissue culture requires prolonged adaptation and it is, therefore, not suitable for diagnosis.
Molecular biology techniques, for example those based on reverse transcription of viral RNA and its amplification by PCR, allow detection of very small amounts of viral RNA from any kind of clinical or environmental sample. HAV infection can be detected by nucleic acid testing methods as provided in the present invention.